In electrophotographic (EP) printers and copiers, an image formed from developed thermoplastic toner is transferred to media such as paper, cardstock, labels, or transparencies. The toner is then fused to adhere the toner to the media. Current EP printers and copiers generally employ one of two types of fuser systems to adhere the toner to the media: a roll fuser system or an instant-on belt fuser system. Both fuser systems use a combination of temperature and pressure to melt and bond the thermoplastic toner either into the fibers of the paper or, in the case of transparencies, onto the surface of the film.
The first type of fuser system, the roll fuser system, comprises two rolls, of which either one or both has an elastomeric coating. Usually the bottom roll, typically known as the back-up roll (BUR), is spring loaded into the top roll, or hot roll (HR), although rolls with fixed centers are also used to create interference between the two rolls. The HR is the roll that contacts the unfused toner. The spring load, or interference in the case of fixed centered rolls, along with the elastomeric coating creates a nip, an area of high pressure and temperature that serves as the working area of the fusing system. The time that the media spends in the nip is known as the residence time or dwell time and is determined by the nip width and process speed of the media. Heat is typically provided by a lamp such as a halogen type lamp. The lamp is usually placed inside the HR, although it may be placed in both rolls or only in the BUR.
The second type of fuser system, the instant-on belt fuser system, comprises a polyamide belt, sometimes called a sleeve, and a soft silicon-coated back-up roll that are pressed together at a particular pressure to form a nip. This system may be an idling-belt system, in which the back-up roll is driven by a drive mechanism and the belt idles, or a driven-belt system, in which the belt is driven by a drive mechanism and the back-up roll idles. The belt in this system is thin, typically 0.10 mm or thinner. This system is called an "instant-on" fuser, because the thin belt wraps directly over the heating element, typically a ceramic type, at the nip, whereby the fuser reaches operating temperatures very quickly due to the lack of thermal mass between the heating element and media.
The roll fuser system is well known and in wider use than the instant-on belt fuser system. However, it has a few inherent drawbacks, especially in color EP applications. One problem with roll fusers is associated with the geometry of the rolls. In order to achieve acceptable fuse grade at faster speeds, a larger nip is needed to meet the required residence times. A common way to make the nip larger is to make the rolls larger. As the exit radius of curvature is increased, however, the release of the media from the surface of the hot roll becomes more difficult; there is a point at which the beam strength of the media is not great enough to overcome the adhesion force of the toner against the hot roll and the media follows the roll rather than properly releasing from the roll. Multi-colored prints are even more difficult to release due to the added pile height associated with mixing of the toner to make non-primary colors. Added pile height increases the tendency for the toner and therefore the media to stick to the hot roll.
A typical solution to prevent sticking of the media to the hot roll is to add silicone oil to the hot roll, thus providing a weak boundary layer between the toner and the hot roll. Another advantage of silicone oil is that the weak boundary layer results in a smooth toner surface, which results in glossy images on paper and, more importantly, vibrant colors when projecting transparencies. Silicone oil, however, has several disadvantages. When oiled sheets are duplexed, oil is transported back through the machine, which may be detrimental to the EP process. Oil supply items can significantly increase the printed cost per page. The oil supply such as an oil roll, web, or oil bottle must be routinely replaced by the user, typically every five-to twenty-thousand sheets. Oil can leave a spotty residue on the sheet, and in the case of transparencies, residual oil can cause the sheets to stick together. Minimizing the quantity of oil metered to the printed sheet is challenging, because it is difficult to ensure consistent oil flow in all situations.
The belt fuser system is advantageous over the roll fuser system in that it allows for a sharp exit angle, which helps the release of the media from the belt without the need for silicone oil. The sharp release angle is formed either by wrapping the belt around a fairly sharp portion of the heater housing or allowing the belt to slacken at the exit portion. The beam stiffness of the media can thereby overcome the tendency of the media to follow the belt. The toner surface, however, is left rough, resulting in a matte finish and a non-translucent transparency that results in non-vibrant or `muddy` looking colors when projected. This `muddy` appearance is caused by scattering of the projector light by the non-flat toner surface.
Another disadvantage of the roll fuser system is the slower process speed needed for color transparencies. Because the optical properties of color transparencies are critical, the toner layers must be well mixed and the surface of the toner must be optically smooth with little or no voids and irregularities. To meet these requirements, the process speed for transparency fusing is slowed, so that the residence time can be increased and therefore more heat can be transferred to the toner. The paper-to-transparency speed ratio for color EP printers is typically 3 or 4 to 1, but some machines have ratios as high as 11 to 1. Also, the fuser temperature is increased so that the energy transferred to the transparency is much greater than for paper applications. Other disadvantages of the roll system compared to the instant-on belt fuser system are that warm-up time is longer and temperature swings are greater.